The present invention is concerned with neutron absorber articles and structures for containing fissile material. It is especially directed to those neutron absorber elements, and the structures that they are incorporated into, which are designed for out of reactor uses, such as, the shipping and/or storage of fissile materials.
In the shipping and storage of nuclear reactor fuel elements and assemblies, which contain large quantities and/or enrichments of the fissile material, U.sup.235, it is necessary to assure that criticality is avoided during normal use, as well as under potential accident conditions.
For example, fuel shipping containers are licensed by the NRC (Nuclear Regulatory Commission) to ship specific maximum fuel enrichments (i.e. weights and weight percent U.sup.235) for each fuel assembly design. In order for a new shipping container design to receive licensing, it must be demonstrated to the satisfaction of the NRC that the new container design will meet the requirements of the NRC Rules and Regulations, including those defined in 10 CFR 71, Appendix B, which is hereby incorporated by reference. These requirements define the maximum credible accident (MCA) that the shipping container and its internal support structures must endure and maintain the subcriticality of the fuel assemblies it holds. Normally the NRC requires an actual 30-foot free fall drop test of a new loaded shipping container, puncture of the shipping container shell, exposure to 1475.degree. F. for 1/2 hour, followed by immersion in water for 8 hours. This, of course, entails destruction of the container so tested.
Criticality is defined as having a sufficient mass of fissionable material in a given configuration to produce a self-sustaining neutron fissioning chain reaction. In a nuclear reactor, controlled criticality is required for power generation. However, outside of a nuclear reactor, criticality is to be avoided. It is well known that criticality can be avoided by reducing the mass of fissionable material, changing its configuration, increasing the distance between fissionable masses, and/or placing shielding (i.e. neutron absorbing materials) between fissionable masses. In nuclear fuel assembly shipping containers, all of the above parameters are used to avoid criticality. In a conventional shipping container, two fuel assemblies are held side by side on an internal support frame. A portion of the support frame is interposed between the two fuel assemblies, and contains two neutron absorber plates, each having a length and width substantially equal to the length and width of the fuel assemblies and a thickness of about 0.19 inches. In the past, these absorber plates have been, for example: an AISI 304L austenitic stainless steel containing at least about 1.3 wt. % natural boron; copper; or carbon steel. These container internals, including the absorber plates, must also be evaluated against the MCA conditions.
Fuel enrichments have been increasing as utilities increase fuel assembly discharge burnup and extend fuel cycle lengths. It is therefore desirable that fuel enrichments greater than current fuel shipping container licensed limits be shipped.
Neutron absorber elements also find use in the storage and industrial handling of fissile materials, where they are used to help limit radiation exposure to workers as well as avoid criticality incidents.
The design, manufacture and use of porcelain enamels, as well as refractory metallic-ceramic coatings are well known to those of ordinary skill in the art of metal coating and is exemplified by teachings found in: Kirk-Othmer, "Encyclopedia of Chemical Technology", Third Edition, Vol. 9, published by John Wiley & Sons, (1980), pp. 1-20, and Metals Handbook Ninth Edition, Vol. 5, "Surface Cleaning, Finishing, and Coating," published by the American Society for Metals, (1982), pp. 509-531, both pertaining to porcelain enameling; and Military Specification "Coating, Metallic-Ceramic" MIL-C-81751B, Jan. 17, 1972, pertaining to cermet coating. All three of the preceding documents are hereby incorporated by reference.
The cermets defined by the aforementioned Military Specification are industrial cermets composed largely of the refractory ceramics aluminum oxide and/or zirconium oxide and metallic aluminum and/or zirconium, with significant quantities of other oxides used as binders, such as borated glass. Minor levels of additional constituents found in the clay additions utilized, are also present. These cermets are used in applications requiring heat resistant or chemical resistant coatings such as jet exhausts or heat exchangers. Coating a steel substrate that provides shape and strength is a relatively simple spraying and fusing process which can be performed using industrial equipment and techniques that are well known to those of ordinary skill in the arts of porcelain enameling and cermet coating.